Black algae stain on asphalt shingles is usually caused from shading of trees that are above the roof, with very little sunlight being able to pass through the trees.
These stains are somewhat very uniform over the stained area and are not streaky as is the case of bleed-through stain on shingles. These black algae stain on your asphalt shingle roofing are dead organic debris that is caused by an alga which is commonly known as Gloeocapsa mamga.
To clean this black algae stain from your asphalt shingle roofing you will have to clean all the debris off the roof and then cut and trim the branches that are overhanging onto the roof in order to expose the roof to more sunlight.
You will then have to spray some oxygen bleach onto the stained areas of the roof and leave it on for around half an hour before washing it off. To determine the amount of water to the amount of oxygen bleach you should mix you will have to refer to the manufacturers directions.
To prevent these algae from re-growing you will have to install some copper or zinc strips under the row of shingles that is closest to the peak of the roof, but be sure and leave a few inches of the lower edge exposed.
This is so that when the rain falls it will wash tiny particles of the copper or zinc metal down the roof killing any algae and removing any stain that may be trying to grip onto your asphalt shingle roofing.
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